A lot of them dont have one at all. The value in their business is having a lot of people using their website (twitter for example). Twitter has a very limited way of making money as of now, yet the company is valued at billions of dollars. Its the ability to reach a massive amount of people that's valuable.
... but this isn't necessarily valuable until a company sells, right?
If you have 100,000 users on your website, and no way of making money , you're in pretty good position. Some brainstorming and figuring out how to implement ads and you're set.
If you have 1,000 users and a great way of making money, its not that valuable.
Also, users tend to like sites with less ads as opposed to more. Its better to build up your userbase before monetizing it.
I think this is one of the keys to success, thank you Jsarokin.